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Incorporation of Spatial Variations in Elastic Rock Properties in Geomechanical Reservoir Models – Workflow and Case Studies


The aim of this work is to utilize seismic velocity data as proxy for vertical and lateral changes in elastic rock properties. These properties, i.e., Young's Modulus and Poisson's Ratio, are used to populate geomechanical finite element (FE) models in order to quantify the effect of the spatial variations in mechanical parameters onto the orientation and magnitude of the present-day stress field. The corresponding workflow improves the layer-cake approach commonly used in geomechanical reservoir modeling as it allows for a more realistic model description through the additional integration of lateral variations in rock properties. In order to test the applicability of the workflow two case studies with 3D seismic surveys were chosen. The first case study is located in the Oberrheingraben, Germany, whereas second utilizes data of the CO2CRC-Project in the Otway Basin, SE Australia. As initial input compressional wave velocities are required which can be inferred, for example, from earlier processing steps or from the velocity model used for depth conversion. From this information a velocity cube is created for the entire survey area. In the next step cubes for shear-wave velocity and density are calculated from empirical relationships. From these three parameters, elastic rock properties with their variations in three dimensions are inferred. However, it should be noted that Young's Modulus and Poisson's ratio created from seismic velocities reflect the dynamic rock properties. For geomechanical modeling, however, the corresponding static values are mandatory, because - among others - the strain magnitudes during static measurements are more in the order of geomechanical relevant processes and static moduli predict the behavior of rock under in situ conditions more accurately. As for both case studies no specific laboratory data are available, empirical relationships were used to convert the dynamic to static properties. In the final step, the data cubes for the static Poisson's Ratio and the static Young's Modulus are transferred to a 3D grid, containing all lithostratigraphic horizons and zones of interest. This grid is then used for stress calculations in the FE simulations.